1. Field of the Invention
The present invention relates to a semiconductor memory device, and is applied, for example, to an MRAM (magnetic random access memory).
2. Description of the Related Art
Conventionally, MRAMs that make use of a magnetoresistive effect of a ferromagnetic body have attracted attention more and more as next-generation solid nonvolatile memories that are capable of achieving high-speed read/write, an increase in capacity and an operation with low power consumption. In particular, MRAMs have attracted special attention since it was found that a magneto-resistance effect element with a magnetic tunnel junction (MTJ) has a high magneto-resistance variation ratio.
The magnetic tunnel junction has, as a basic structure, a three-layer stacked structure that comprises a free layer (magnetization free layer), the direction of magnetization of which is easily changed, a pinned layer (magnetization fixed layer) which is opposed to the free layer and maintains a predetermined direction of magnetization, and a tunneling barrier layer (insulation layer) which is interposed between the free layer and the pinned layer. In the magnetic tunnel junction, an electric current flows, tunneling through the tunneling barrier layer. At this time, the resistance of the junction varies in accordance with relative angles of the magnetization directions of the pinned layer and the free layer. When the magnetization directions are parallel, the resistance takes a minimum value. When the magnetization directions are antiparallel, the resistance takes a maximum value. This variation in resistance is called “tunneling magneto-resistance effect” (hereinafter referred to as “TMR effect”). In a case where a magnetic element having a magnetic tunnel junction is actually used as one memory cell of the MRAM, the parallel/antiparallel state (i.e. minimum/maximum resistance) of the magnetization directions of the pinned layer and free layer is associated with binary information “0” or “1”, thus storing information.
In a write operation of magnetic information, only the magnetization direction of the free layer is reversed. In a read operation of magnetic information, a sense current is let to flow through the memory cell, and the variation in resistance due to the TMR effect is detected. The above-described magneto-resistance effect element that makes use of the above-described TMR effect is hereafter referred to as “MTJ element” (magnetic tunnel junction element).
In the meantime, as a greater number of MTJ elements are integrated in order to realize a giga-bit (G bit) class MRAM, a write current that is necessary for writing data in the MTJ elements increases accordingly.
If heat produced by the write current (hereinafter referred to as “current-passage heat”) is conducted to a non-selected MTJ element, the spin of the non-selected MTJ element is reversed and erroneous write (disturb) occurs. It is thus necessary to prevent the current-passage heat from being conducted to the non-selected cell.
Moreover, if the current-passage heat remains in the selected cell for a long time after the completion of a write operation, the write speed would decrease. It is thus necessary to quickly radiate the current-passage heat from the selected cell immediately after the write operation.
On the other hand, in recent years, studies have been made of MRAMs of a write scheme in which only the magnetization direction of the free layer is reversed by making use of heat assistance of the current-passage heat to the memory cell (see, for instance, Jpn. Pat. Appln. KOKAI Publication No. 2003-298025).